The present invention relates to a method and apparatus for sound reproduction, and more particularly to a load and dispersion cell that provides for more uniform frequency dispersion when used in combination with a variety of transducer types.
Known technology for sound reproducers depends heavily on dynamic speaker cones in sealed and ported cabinets, and on bipolar radiators when large, light diaphragm surfaces are involved. In the following specification, when the word "transducer" or "transducer means" is used, it is intended to refer generally to transducers, diaphragms, cones, piston arrays and the like used for sound reproduction.
Conventional loudspeaker designs are often based on a high-pass filter model. In such designs dynamic loudspeaker enclosure combinations of sealed or vented design are provided in which the mass of the transducer essentially provides one of the system masses and is only secondarily influenced by an air mass in contact with the diaphragm on both sides. If venting or tuning is used with an enclosure it is accomplished by way of an opening separate from that into which the transducer is placed. The venting can take the form of a tuning duct, a mass attached to a second passive diaphragm, a highly lossy opening, an acoustic transmission line, etcetera.
There exists a family of transducers with diaphragms of negligible mass, large active area and small peak displacement. They are generally regarded as being unsuitable for mounting in cabinets of the above types, as this would raise the low frequency cut-off of the system to an unsatisfactorily high frequency for a cabinet of reasonable size. This has led to the use of light and large diaphragm transducers as bipolar assemblies (without enclosure) primarily to preserve maximum use of the low frequency response. Bipolar systems, however, usually suffer from cancellation at low frequencies between the front and rear radiation of the transducer.
The light, large diaphragm has inherent advantages. Because of the low mass and simple structure, it can respond to low as well as very high audio frequencies cleanly and smoothly. Since a dynamic/cone type transducer of equal area has a heavier diaphragm and cannot normally reproduce high frequencies, a smaller and lighter cone must be utilized to take over from the larger cone.
The inherent wide-band performance-capability and time alignment makes light, large diaphragm transducers attractive. As well, such transducers provide an absence of cross-over frequency phase-aberration and no need for time alignment between drivers. Large diaphragms of any transducer type, however even if they can reproduce the necessary wide frequency range suffer from high frequency beaming.
As frequencies get higher and higher, the dispersion of frequencies becomes narrower and narrower until at high frequencies, only a narrow beam of high intensity remains. This makes for unsatisfactory dispersion when used for monophonic as well as stereophonic reproduction. One approach to overcome this is to make the transducer dimensions smaller in the direction in which dispersion is required. Another is to use an acoustic lens. But making the transducer smaller sacrifices low frequency sound level capability while using an acoustic lens adds cost. Alternatively, attempts have been made to use many transducers positioned along a curve (or on a sphere). The circular radiating field created by this approach leaves gaps and phase-aberrations at the beam intersections. Good performance is difficult to achieve, and takes up a lot of space.
Another approach has been to use a flat transducer panel with segmented drive from delay line taps to simulate a point source lying some distance behind the transducer assembly. This is a relatively complicated solution and requires expensive, sophisticated equipment. Another approach is to use transducers of different widths for different frequencies. This sacrifices the elegance of the wide bandwidth capability of the light transducer panel system and introduces the cost and problems of cross-over networks and tends to increase the size of the structure.
Normally line sources with open transducers/diaphragms/cones or flat panels do not give sufficient high frequency dispersion unless the transducers are about one wave length or less wide at the highest frequency of concern. A reduction in transducer width to accommodate high frequency dispersion however leads to reduced capability to reproduce low frequencies since normally transducer active area and displacement must be sacrificed.
Mr. G. A. Briggs, in a paper entitled "All About Audio and High-Fi--The Listening Ear", published in "Hi-Fi Annual and Audio Handbook" - 1959 Edition, at pages 26 and 27, describes and illustrates the use of a Kolster-Brandes Ltd. slot diffuser consisting of a slotted board a bit larger than a speaker cone, which is placed in front of a large sized dynamic speaker with the slot along the vertical diameter. The slot widens the dispersion of the high frequency energy, emanating only from the central area of the large cone, horizontally to a fan shape. (The sound energy would otherwise be concentrated in a narrow pencil beam). The board as described is spaced from the speaker cabinet leaving an air gap through which low frequency sound energy can escape and thereby minimize the effect of the slotted board on the low frequency performance of the driver.
Another reference of general background interest is Nakanishi U.S. Pat. No. 4,280,585 issued July 28, 1981 in which an array of speaker units are vertically mounted in a cabinet with a plurality of reflecting boards and sound pressure guide boards so that sound from the speaker units is passed to openings in the front of the enclosure.
It is an object of the present invention to provide a load and dispersion cell and method which will permit usage of transducers of larger width while at the same time providing improved dispersion of sound at higher frequencies without sacrificing low frequency capability and allowing it even to be extended. It is another object of the present invention to provide an economical and simple cabinet construction incorporating the load and dispersion cell which will provide such features when used in conjunction with such transducers.